This invention relates to the sensing of fluid flow through a tubular channel. In particular, the present invention concerns flow sensing devices which detect a pressure differential created by an obstruction in the channel and translate that pressure differential into a measure of the fluid flowrate through the channel. The present invention is particularly useful in connection with medical ventilator devices.
The term "fluid" includes both gases and liquids. A fluid flowrate is the volume of fluid passing a location during a given amount of time. The accurate sensing of fluid flow is an important parameter for many medical mechanical and chemical systems.
A common example of a device for sensing fluid flow includes a channel having a rigid flow obstruction which provides a flow aperture. Passage of the fluid through the reduced aperture created by the obstruction reduces the pressure of the fluid. The difference between the fluid pressure upstream of the obstruction and immediately downstream of the obstruction is known as the pressure differential. The amount of pressure differential is related to the fluid flowrate. A small flowrate yields a small pressure differential and a large flowrate yields a large pressure differential.
Typical flow sensing devices provide a port in the channel immediately upstream and immediately downstream of the flow obstruction. The pressure at these ports is sampled and compared by means of a pressure transducer. The differential pressure may be compared to the fluid flow and displayed by any of a number of means, including a simple calibrated meter or a microprocessor with an internal look-up table
An inexpensive, easily-applied and commonly used flowsensing obstruction is the fixed orifice. This obstruction commonly takes the shape of a thin, metal or plastic plate, with a sharp-edged hole, and is installed between flanges in the flow channel, usually so that the hole is concentric with the channel. The fixed orifice causes the fluid flow stream to converge to much the same shape as that obtained by a venturi tube or flow nozzle. The flow stream continues to converge a short distance downstream from the orifice plate, then diverges back to the full channel diameter. The point of smallest flow cross-section, and the point of lowest pressure, is termed the vena contracta.
A pressure differential device may be connected to ports both upstream and downstream from the orifice. Conventional port locations are one channel diameter upstream from the orifice and downstream at the vena contracta.
The correlation between the total volumetric fluid flow and the pressure differential across a fixed orifice is well known in the art. For example, this relationship is explained in T. Baumeister, E. Avallone & T. Baumeister III, Marks' Standard Handbook for Mechanical Engineers, section 16 at 15-16 (8th ed. 1978). It is important to note that a fixed orifice flow meter normally exhibits a square-law relationship between the pressure differential across the orifice and flowrate through the orifice. In other words, under constant system pressure and enthalpy conditions, the pressure differential across the orifice is proportional to the square of the fluid flowrate through the orifice hence through the channel. For example, the maximum measurable flowrate through a fixed orifice represents a given pressure differential across the orifice. This pressure differential is termed the maximum pressure differential. Similarly, the minimum measurable flowrate through a fixed orifice represents a unique pressure differential across the orifice. This pressure differential is termed the minimum pressure differential.
The "turndown ratio" is the ratio between the maximum measurable flowrate through the orifice and the minimum measurable flowrate through the orifice. Since the maximum and minimum measurable flowrates represent specific pressure differentials across the orifice, the turndown ratio may also be expressed as the ratio between the maximum pressure differential and the minimum pressure differential.
A low turndown ratio presents a problem in flow sensing apparatus. This problem is discussed in Silverwater U.S. Pat. No. 4,688,433, which explains that low pressure differentials must be sensed with great accuracy in order to provide a meaningful flowrate indication. At low flowrates, the pressure differential is exceedingly small and difficult to measure. Also, as discussed in Billette, et al. U.S. Pat. No. 4,006,634, the error at lower flowrates is a greater percentage of the flowrate than the error at higher flowrates.
A device for measuring fluid flow is disclosed in Billette, et al. U.S. Pat. No. 4,006,634. The Billette, et al., reference shows a variable orifice flow meter having an obstruction comprising an outer rim portion, a plurality of flexible leaves extending radially inwardly therefrom, and an inner orifice portion. Under low flow, the inner orifice is relatively small and, therefore, the area of obstruction is relatively large. As the pressure of the fluid increases, the leaves begin to flex, thereby decreasing the area of the obstruction and enlarging the area of flow. The Billette device is directed to overcoming the problems of the fixed orifice flow meter with regard to low turndown ratio and errors at low flowrates.
Another design of a variable orifice fluid flow sensing apparatus is disclosed in Osborn U.S. Pat. No. 4,083,245. That reference shows an obstruction disposed about the periphery of a housing, the obstruction having a cut-out flap portion hingably connected thereto. This flap bends open with increased fluid flow so as to increase the effective flow area.
Notwithstanding their improvements over the prior art fixed orifice flow sensing device, both the Billette, et al., and Osborn designs suffer susceptibility to build-up of contaminants about the periphery of their obstructions. This build-up is detrimental to the accuracy of these devices.
Another example of a variable orifice flow sensing device is shown in Silverwater U.S. Pat. No. 4,688,433. That reference discloses a U-shaped rigid member disposed downstream of a thin, circular disk. The rigid member is mounted symmetrically within the conduit by means of a pin attached at both ends to the conduit interior. This design substantially reduces the likelihood of the build-up of contaminants. On the other hand, this design has its own drawbacks. The Silverwater device acts as a variable orifice flow meter until the flow reaches a certain fixed maximum, at which point the flexible disk is flattened against the rigid member, thereby resulting in a fixed orifice. This fixed orifice has an effective flow area substantially less than that of the housing itself. Thus, at high flowrates, the Silverwater device provides high resistance to flow, and a concomitantly high pressure differential.
The problems associated with a fixed orifice flow meter are compounded by the build-up of contaminants against the rigid orifice plate which defines the restricted aperture. The presence of an area permanently perpendicular to the flow path within the channel, alters the flow stream so as to allow foreign particulate matter entrained in the flow to deposit on the permanently perpendicular area. This build-up of contaminants can alter the flow characteristics through the apparatus by adding mass to the obstruction assembly or obstructing the pressure sensing ports, thereby changing the relationship between the flowrate and pressure differential. A slight change in this relationship will render the calibration of the device incorrect and reduce the accuracy of the device.
The monitoring of flowrates and the volume of fluid transferred is critically important in medical ventilators. For example, the volume of air transferred into the lungs must be accurately monitored to: (1) ensure compliance with the orders of the attending physician; (2) provide a basis for ventilator settings so as to optimize arterial blood gases; (3) provide an assessment of the patient's ability to support unassisted ventilation; and (4) ensure delivery of an adequate volume of air to the lung to prevent a partial collapse of the lungs. In addition to the foregoing reasons, the volume of air exhaled from the lungs must also be accurately monitored to allow an assessment of possible air leaks within the ventilator circuit, endotracheal tube, and the lungs. Therefore, the flow sensing apparatus used in medical ventilators must provide accurate measurements of the flowrate.
However, the flow sensing apparatus used in medical ventilators are subject to a variety of adverse conditions. These conditions include the wide fluctuation of flowrates, the transport of foreign particulate matter in the flow which may foul the flow sensing apparatus and provide dangerous misreadings, and the need to regularly clean and sterilize the flow sensing apparatus.
In addition, the medical flow lines to which flow sensing apparatus are attached are often a substantially different diameter than the channel of the flow sensing apparatus. The sudden change in the diameter of the flow path between the medical line and the flow sensing apparatus creates pressure waves within the flow stream which adversely effect the accuracy of the pressure measurements.
Accordingly, there exists a need for a flow sensing apparatus that has a flow turndown ratio which is high, and provides a relatively high pressure differential at low flowrates and a relatively low pressure differential at high flowrates, that is, a pressure turndown ratio which is high, an improved accuracy of readings by reducing the presence of pressure waves in the flow stream and reducing the build-up of contaminants on the apparatus, and sufficient durability to withstand the necessary maintenance of regular sterilization procedures.